Re: [PATCH RFC] x86/speculation: Clarify Spectre-v2 mitigation when STIBP/IBPB features are unsupported

[Borislav Petkov <bp@alien8.de>]

On Tue, Jan 21, 2020 at 05:02:57PM +0100, Vitaly Kuznetsov wrote:
> When STIBP/IBPB features are not supported (no microcode update,
> AWS/Azure/... instances deliberately hiding SPEC_CTRL for performance
> reasons,...) /sys/devices/system/cpu/vulnerabilities/spectre_v2 looks like
> 
>   Mitigation: Full generic retpoline, STIBP: disabled, RSB filling
> 
> and this looks imperfect. In particular, STIBP is 'disabled' and 'IBPB'
> is not mentioned while both features are just not supported. Also, for
> STIBP the 'disabled' state (SPECTRE_V2_USER_NONE) can represent both
> the absence of hardware support and deliberate user's choice
> (spectre_v2_user=off)
> 
> Make the following adjustments:
> - Output 'unsupported' for both STIBP/IBPB when there's no support in
>   hardware.
> - Output 'unneeded' for STIBP when SMT is disabled/missing (and this
>   switch_to_cond_stibp is off).
> 
> RFC. Some tools out there may be looking at this information so by
> changing the output we're breaking them. Also, it may make sense to
> separate kernel and userspace protections and switch to something like
> 
>   Mitigation: Kernel: Full generic retpoline, RSB filling; Userspace:
>    Vulnerable
> 
> for the above mentioned case.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  Documentation/admin-guide/hw-vuln/spectre.rst | 3 +++
>  arch/x86/kernel/cpu/bugs.c                    | 9 +++++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)

There's another attempt to fix similar aspects of this whole deal going
on ATM:

https://lkml.kernel.org/r/20191229164830.62144-1-asteinhauser@google.com

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette